Spelcnmr Coupling

In order to enhance the sample concentration injected onto the column for HPLC separation, solid-phase extraction (SPE) can be used. In online SPE, the sample is injected onto a cartridge that replaces the loop of the injection device of the chromatographic system. By switching the valve, the enriched sample is transferred directly onto the separation column. The advantage of online SPE-LC coupling is the direct enrichment of analytes on a cartridge in the HPLC system excluding light and oxygen.

A new online SPE-LC technique in which both the trace enrichment and the separation are performed on one single short HPLC column (10-20 mm in length) is called the SSC (single short column) approach. By performing online sample enrichment before LC-NMR analysis, analyte detectability may be significantly improved. The hyphenation of this approach with NMR spectroscopy can reduce the effective analysis time necessary for separation and structure elucidation. As in conventional LC-NMR coupling,

stopped-flow as well as continuous-flow experiments are possible. The higher concentration of sample is advantageous for continuous-flow LC-NMR experiments. For online SSC-NMR, 10 mL of a 5 |gmL_1 mixture was loaded on to a 12.5x4.6 mm i.d. C18 short column and separated on-column with the eluents (A) acetonitrile-0.05% phosphoric acid and (B) water-0.05% phosphoric acid 30:70 (v/v) at a reduced flow rate of 0.3 mLmin-1 and transferred to the NMR spectrometer. Figure 8 shows the contour plot of the separation of ibuprofen, fenoprofen and naproxen.

In the case of these drugs, the differences between the structures can be determined by the signals in the aromatic region, so the structure of the separated compounds can easily be assigned by the aromatic resonances. Whereas naproxen is a substituted naphthalene derivative, fenoprofen and ibuprofen both contain a phenyl ring which is meta-substituted in the case of fenoprofen and para-substituted in ibuprofen. For unambiguous structural elucidation, different rows could be extracted from the contour plot to obtain conventional 1H NMR spectra, as shown in Figure 9 for ibuprofen.

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